How to tell: You are trying to use the brush or eraser but the outcome is not as you expected.

How to fix: Check whether the layer you are painting on has a Layer Mask. If there is, you may be painting on the Layer Mask instead of the Layer itself. In that case click the Layer to make that the active painting area.

Another similar possibility is that a Channel is active instead of a Layer. In that case click a layer in the Layers Dialog to make a layer active.